Silicone rubber, i.e., high molecular weight, vulcanized polydiorganosiloxane, is able to withstand wide temperature variations without an appreciable effect on its physical properties. Silicone rubber is virtually unaffected by ultraviolet radiation, even over long periods of time. It is also resistant to ozone, oil, salt, water and other road and automotive chemicals. Silicone rubber has been used in windshield wiper blades, gaskets, spark plug boots and weather stripping.
However, silicone rubber articles currently existing in the art tend to leave an undesirable coating, or haze, on surfaces in which they come into contact, such as windshields. The coating is caused by the leaching of a low molecular weight silicone polymer process fluid, used to aid in the manufacture of conventional silicone rubber articles. In the case of silicone rubber wipers the process fluid leaches onto the windshield at an unacceptably high rate, yielding a vision-obstructing haze. Experiments indicate that a slower leaching process fluid would not create an undesirable haze, but rather would confer beneficial hydrophobic qualities and anti-hazing properties to the windshield surface.
Additionally, other than for sponge or foam applications, the prior art teaches the use of a single peroxide, either a "high pressure" or a "low pressure" peroxide, as the vulcanization catalyst for the high molecular weight polydiorganosiloxane base. Low pressure peroxide catalysts include diaroyl peroxides such as 2,4-dichlorobenzoyl peroxide. High pressure catalysts include dialkyl and diaralkyl peroxides such as dicumyl peroxide. Formulations using only one peroxide as a catalyst cause a less than optimal amount of crosslinking, with less of a physical barrier to the leaching of low molecular weight silicone process fluid. Wiper blades cured with one catalyst therefore have less than optimal anti-hazing properties.
Use of both a low-pressure and a high-pressure catalyst could theoretically induce an increased degree of crosslinking or cure, and a concomitant increase in elasticity, and resiliency, and decrease in tackiness and compression set. One of the perceived problems in the prior art of using both a "high pressure" and "low pressure" catalyst is that blistering or bubbling by the low-pressure catalyst byproducts will occur at the high temperature needed to "kick off" the high-pressure catalyst, such as 340.degree. F. While such a reaction is desirable for sponge or foam applications, it is not desirable for articles requiring more structural integrity, such as windshield wiper blades, spark plug boots, weather stripping and gaskets.
A long felt need continues to exist for an improved silicone rubber article that confers beneficial hydrophobic qualities and possesses slow-leaching or anti-hazing properties. A need further exists for silicone rubber articles having an increased degree of crosslinking or cure.